In many instances, plastic containers are prepared or manufactured by molding each half of the container and subsequently welding those two halves together. This results in a seam that extends around the container.
In the manufacture of battery jars, for use in producing acid batteries, one procedure used to mold jars is to mold half jars with each half containing three side walls and a bottom wall which are sometimes referred to as flanges. Generally, the bottom and side walls are molded so as to be longer than needed and are ultimately trimmed to a proper dimension so that the welding together of two such jar halves will produce a battery jar having the length needed to produce the desired size battery jar.
When the two halves are properly trimmed and subsequently hot-plate welded together, the opposing edges of each half are heated to the point that the edge area becomes molten. This heating results in a relatively thin band in the wall area that becomes warmed outside the immediate joint area. The wall outside of that relatively narrow band, however, remains cooler. Expansion of the plastic material induced as a result of the heating along that band will be in an outward direction, from the center of the walls thickness toward the sides thereof, tending to make that side wall thicker as well as in a lateral direction or normal to the joint. However, because of the relatively cooler area outside that narrow heated band axial expansion along the joint or in the direction of the joint will be retarded.
Following heating, the two halves are pressed together and a beaded weld joint will be formed. On cooling, shrinking will occur both laterally, with respect to the joint, and inwardly, with respect to the wall thickness. Because expansion was experienced in both of these directions, subsequent shrinking of material on cooling will substantially only return the material to its original dimensions. However, because there was initially compression occurring along the joint, caused by the fact that material was trying to expand but was being retained by the relatively cooler side wall area behind the joint, shrinkage will occur axially along the joint. This axial shrinkage along the joint produces tensile stresses in the finished joint which the side wall tends to try and correct for by bowing the wall inwardly or outwardly, depending upon the balance of other parameters about the open end or top of the completed battery jar. It can be appreciated that such bowing of walls inwardly or outwardly is undesirable from the standpoint of making it difficult to correctly fix covers to the battery jars and also where the walls bow inwardly, it is sometimes difficult to correctly insert the lead battery elements into the jar.